Superconducting devices, such as slip-rings and homopolar motors/generators

ABSTRACT

A device {i.e., a slip-ring or a homopolar motor/generator) ( 40, 50, 80 ) is adapted to provide electrical contact between a stator and a rotor ( 41, 83 ), and includes: a current-carrying brush-spring ( 31, 84 ) mounted on the stator, and having two opposite surfaces; a fibrous brush assembly ( 35, 69 ) mounted on the conductor, the brush assembly having a bundle of fibers ( 36, 71 ) arranged such that the tips of the fibers will engage the rotor for transferring electrical current between the stator and rotor; a ribbon ( 33, 85 ) of superconducting material mounted on each opposite surface of the current-carrying brush-spring and communicating with the stator and the brush assembly; and another ribbon ( 29, 86 ) of superconducting material mounted on the rotor. The device is submerged in a cryogenic fluid at a temperature below the transition temperatures of the superconducting materials such that the electrical resistivity of the device will be reduced and the current-transfer capability of the device will be increased.

TECHNICAL FIELD

The present invention relates generally to electrical contacts for usewith slip-rings, homopolar motors/generators, etc. that are used totransmit electrical power and/or signals between two members (e.g., arotor and a stator), and, more particularly, to improved devicesutilizing fiber-on-tips (“FOT”) brush technology and having ribbons ofsuperconducting material(s) that have reduced electrical resistivity(ies) when cooled below their transition temperature(s) and that maytransmit greatly-increased levels of electrical current between themembers when so cooled.

BACKGROUND ART

Electrical contacts are used to transfer electrical power and/orsignal(s) between a rotor and a stator. These devices are used in manydifferent military and commercial applications, such as solar arraydrive mechanisms, aircraft and missile guidance platforms, wind energysystems, computed tomography (“CT scan”) systems, and the like. In someapplications, slip-rings are used in conjunction with other components,such as torque motors, resolvers and encoders. Electrical slip-ringsmust be designed to be located on the platform axis of rotation, or bedesigned with an open bore which locates the electrical contactsoff-axis. Hence, the designations “on-axis” and “off-axis” slip-rings,respectively.

The diameters of slip-rings range from a fraction of an inch to multiplefeet, and the relative angular speed (w) between the rotor and statormay vary from as little as one revolution per day to as much as 20,000revolutions per minute (“rpm”). In these various applications, theelectrical contacts between the rotor and stator should: (1) be able totransfer power and/or signal(s) without interruption at high relativesurface speeds, (2) have long wear life, (3) have low electrical noise,and (4) be of a physical size that allows multiple circuits to bepackaged in a minimum volume.

Proper management of the electrical and mechanical contact physicsbetween the stator-mounted brush assembly and the rotor allows demandingrequirements to be met. For example, if the application is an off-axisslip-ring that allows an x-ray tube in a CT scan gantry to rotate aboutthe patient's body, the electrical contacts must be designed to carryabout 100-200 amps (with possible surges of hundreds of amps), tooperate at surface speeds on the order of 15 meters per second(“m/sec”), to last for 100 million revolutions, and to occupy a minimalvolume within the gantry. In order to meet the 100 million revolutionrequirement for a device that is about six feet [1.8288 meters (“m”)] indiameter, the brush force (i.e., the force with which the brush tips areurged against the rotor) must be low to minimize frictional heating andyet maintain a large number of contact points between the brush androtor ring to achieve the required current density.

Various arrangements and configurations of prior art slip-ringsemploying FOT brush assemblies are representatively shown and describedin U.S. Pat. No. 7,105,983 B2, U.S. Pat. No. 7,339,302 B2, U.S. Pat. No.7,423,359 B2, U.S. Pat. No. 7,495,366 B2, U.S. Pat. No. 7,545,073 B2,and in US 2014/0045348 A1 (PCT/US2012/00137, filed Mar. 13, 2012). Theseprior art references are assigned to the assignee of the presentapplication, and are hereby incorporated by reference.

There has been a renewed interest in the use of fibrous metal brushes inrecent years. Metal fiber brushes have the capability of handling highercurrent densities, of having lower electrical noise, and of havinglonger life while operating at higher surface speeds. Each of theseparameters is related to more points of contact between brush and rotorring than with prior art composite-material brushes, to less force perfiber, and to less frictional heating. The actual area of contactbetween the fiber tips and a rotor ring is known as the “interfacial”area of contact. It is known that the actual area of contact between theface of a composite brush and a rotor is much less than the outline ofits projected geometric area. Hence, the reason for sub-dividing brushesinto elements which, in some cases, are individual small-diameterfibers. For example, FOT contacts are capable of carrying currentdensities on the order of 2000-3000 amps/sq.-in. for typical militaryand industrial applications, whereas composite-material brushes arelimited to carrying current densities of about 200-600 amps/sq.-in. forthese applications. FOT brushes can be designed to meet higher currentdensities by increasing the diameter of the bundle, and, thus, thenumber of fibers in the bundle.

Also, it has been shown that brush bundles designed with the centermostfibers removed, will generate less frictional heat. Table 1, below,contains data from Table 2 in US 2014/0045348 A 1, supra, and furthershows the improvement of the improved FOT brushes (i.e., with thecentermost fibers removed) over the prior art FOT brushes (i.e., withoutthe centermost fibers removed). This table shows that the improved FOTbrush generates less frictional heating than the prior art brush by afactor of 82 to 86 times. When frictional and electrical tests arecombined, the factor decreases in proportion to the square of thecurrent. The improvement factors of 22× and 6.1× can be further improvedby increasing the number of fibers in the bundle. In addition to this,the number of brushes can be designed to meet high current requirements.

TABLE 1 Frictional and Electrical Test Results Improved Prior Art FOTBrush FOT Brush Improvement Small Rotor Large Rotor Factor FrictionalHeating Current (amps)  0  0 Surface  8  8 Speed (m/s) ΔT (° C.) 32.9 −30.8 = 2.1 28.3 − 22.4 = 5.9 Frictional 2.1° C. × 3.3 5.9° C. × 101 596/6.93 = Heating (cal.) cal/° C. = 6.93 cal/° C. = 596 86.0x Current(amps)  0  0 Surface 14 14 Speed (m/s) ΔT (° C.) 34.2 − 30.8 = 3.4 31.5− 22.4 = 9.1 Frictional 3.4° C. × 3.3 9.1° C. × 101 919/11.22 = Heating(cal.) cal/° C. = 11.22 cal/° C. = 919 81.9x Frictional and ElectricalHeating Current (amps) 10 10 Surface  8  8 Speed (m/s) ΔT (° C.) 35.4 −32.9 = 2.5 30.1 − 28.3 = 1.8 Electrical 2.5° C. × 3.3 1.8° C. × 101181.8/8.25 =  Heating (cal.) cal/° C. = 8.25 cal/° C. = 181.8 22xCurrent (amps) 20 20 Surface 14 14 Speed (m/s) ΔT (° C.)  44.3 − 34.2 =10.1 33.5 − 31.5 = 2   Electrical 10.1° C. × 3.3 2° C. × 101 202/33.33 =Heating (cal.) cal/° C. = 33.33 cal/° C. = 202 6.1x

An article (Lewis, Norris E., Reed, Charles W., Witherspoon, Barry K.;“Lubrication Quantity: Observation of the Effects on Gold SlidingContact Wear Character and In Situ Contamination Formation”; IEEETransactions on Components, Hybrids, and Manufacturing Technology, Vol.CHMT-2, No. 1., March 1979.), reports:

-   -   “It has been observed that for certain slip-ring assemblies the        wear character is altered by varying lubricant quantity. These        observations were made on gold alloy sliding contacts tested        under closely controlled laboratory conditions and on similar        contacts taken from field installations. For thin-film boundary        lubrication, the wear process follows the adhesive and abrasive        modes set forth by Antler. However, for the flooded boundary        lubrication, the wear process appears to stay in the adhesive        mode as long as the flooded condition exists.    -   The observations that have been made are as follows:    -   1) Independent of the lubricant used, sliding gold contacts        lubricated with thin films generated finely-divided black        appearing wear particles    -   2) Independent of the lubricant used, sliding contacts flooded        with lubricant to the extent that the brush and ring contact        zone are submerged generate much larger wear particles, which        are gold in appearance, than the thin-film lubricated contacts.        . . .    -   3) Sliding contacts that have been flooded with lubricant are        less prone to form contaminant films in the contact zone that        result in electrical noise than those lubricated with thin        films. This is particularly true when the contact assemblies are        exposed to multichemical environments.”

When lubricant levels are increased such that the contact interface issubmerged, the wear mode changes from abrasive to adhesive. Studies haveshown that when adhesive wear occurs, rotor material will transfer fromthe rotor (ring) to the stator (brush). As sliding continues, the ringmaterial becomes workhardened, and the life of the contacts isincreased. When silver/copper (“Ag/Cu”) FOT brushes are run on anelectrodeposited gold rotor ring while submerged in liquid nitrogen,gold from the rotor ring transfers to the fiber tips and, thus, gives amore-noble gold-on-gold contact interface. (See SEM/EDAX results inFIGS. 1A-1D, and FIGS. 4A-4D.)

In addition to this, there is a significant cost reduction by usingsilver/copper alloy fibers with transferred gold tips versus gold alloyfibers. As shown in Table 2, below, and using the pancake-type slip-ringillustrated in FIG. 7, which has sixteen brushes, as a cost-comparisonexample for gold alloy fibers vs. silver/copper fibers with transferredgold tips, the difference is $2,521.60 (i.e.,$2,560.00-$38.40=$2,521.60) for sixteen brushes, based on 4000fibers/brush.

TABLE 2 Cost Comparison for Gold Alloy Fibers vs. Silver/Copper (Ag/Cu)Alloy with a Transferred Gold Tip for the Pancake- Type Slip-Ring withSixteen Brushes Shown in FIG. 7 Gold Alloy Ag/Cu with Gold Tip Number ofCost per Cost for Cost per Cost for Fibers/Brush Brush 16 Brushes Brush16 Brushes 1200 $48.00 $768.00 $0.72 $11.52 2000 $80.00 $1,280.00 $1.20$19.20 4000 $160.00 $2,560.00 $2.40 $38.40

Another article, [Reichner, P. and Doshi, V. B.; “A Homopolar Motor forthe Demonstration of New High Current Brushes”; Advances in ElectricalCurrent Collection: Ed. I. R. McNab. New York: Elsevier/North-HollandInc.; (1982), at pp. 69-71], reports:

-   -   “Under proper conditions, extremely high currents can be        transferred across sliding electrical contacts. This has been        demonstrated in recent research experiments with solid and fiber        brushes as well as with liquid metal current collection systems.        The ultimate objective of this research is the development of        practical electrical machines of high power density and high        efficiency. Liquid metal current transfer systems have the        advantage of essentially 100% area coverage of the slip-ring but        they introduce problems of liquid containment, particularly for        motors where inertial confinement techniques cannot be applied        throughout the full operating speed range. Commonly used liquid        metals also have problems of chemical stability and        compatibility, more stringent insulation requirements and even        safety hazards. The recent experimental results with solid and        fiber brushes promise efficient collector operation with good        brush life.    -   In low voltage high current machines such as the homopolar (or        unipolar) motor or generator, the current collector at        conventional current densities is a major factor in machine        size, weight and power loss. This is seen in the relative size        of the spirally grooved collectors and the active length in the        early machine. . . . Here, the active length is less than 10% of        the rotor length.    -   Translation of experimental high current brush concepts into        practical machine applications requires consideration of and        control of a number of factors, primarily those associated with        temperature and atmosphere. For example, three potential effects        of increased slip-ring coverage are as follows:        -   (1) There is a concentration of power dissipation, which            will result in excessive temperatures unless adequate            cooling techniques are employed.        -   (2) The reduced access time of the track surface to the            surrounding atmosphere may influence the effectiveness of            lubricating films in the form of adsorbed or chemisorbed            surface layers and could result in higher friction and wear.        -   (3) The increased concentration of wear debris, with reduced            area for removal, could interfere with the intimacy of brush            contact and possibly with the freedom of brush motion.        -   In addition, when the brush current density is greatly            increased, the sectional area of the shunt lead wire,            between the brush and the stator conductor, must increase.            The shunt length may also be increased to maintain            flexibility. The shunt and brush holder become major factors            in the current collection system design. Inertial effects            must also be considered in some applications, such as            vehicular drives.”

As summarized in Table 3, below, the electrical contact problems forslip-rings and homopolar motors listed above can be eliminated by theapplication of superconducting ribbons and FOT brushes submerged inliquid nitrogen.

TABLE 3 Solution Liquid Superconducting Problem Nitrogen Ribbons PowerDissipation ✓ ✓ Effectiveness of Lubricating Films ✓ Wear Debris ✓Sectional Area of the Shunt Lead ✓ ✓ Wire High Machine Power Density ✓ ✓

Efforts have been directed toward developing a superconducting homopolarmotor. This would be particularly advantageous in shipboard propulsionsystems. One source [Patel, Makund R.; Shipboard Propulsion, PowerElectronics, and Ocean Energy; CRC Press, Boca Raton, Fla. (2012) (atpp. 219-220)] has recognized the possibility of weight and volumereductions by using a homopolar motor over a conventional copper motor.For example, a 21-megawatt 4-kilovolt copper motor having a shaftrotating at 150 rpm would weigh about 183 tonnes (i.e., 1 metrictonne=1000 kilograms or 2204.6 pounds), whereas a 36.5-megawatt6.6-kilovolt high-temperature superconducting motor having a shaftrotating at about 120 rpm would weigh less than 75 tonnes. No commutatoris needed in the homopolar machine. Because the homopolar motor usesonly DC current, it permits the motor drive to be less complex and lessexpensive than other motor drives. However, high-power homopolar motorswith tens of megawatt ratings must carry enormous currents. This posesdesign challenges in current collection, brush erosion and sparking.While a superconducting homopolar motor can be more power-dense, runquieter, and be more energy efficient than a permanent magnet motor,this article states that the successful collection of large currentsneeded for homopolar motors with tens of megawatts remains to bedemonstrated. [Accord, Superczynski, Jr., Michael J.; “Homopolar Motorwith High Temperature Superconductor Field Windings”; IEEE Transactionson Applied Superconductivity, Vol. 7, No. 2 (June 1997).]

Another source [Kalsi, Swann Singh; “Applications of High TemperatureSuperconductors to Electric Power Equipment”; IEEE, John Wiley & Sons,Inc., Hoboken, New Jersey (2011) (at p. 135)] reviewed the developmentof superconducting homopolar motors, and concluded that brushes are thebiggest challenge to the development of DC homopolar machines. Thisarticle recites that solid carbon and metal-graphite brushes have beenfound inadequate due to their low current density capability andexcessive wear. Liquid metal brushes were reportedly also tried, butwere deemed to be unsuitable due to material and life limitations. Thisarticle reports that copper fiber brushes operating in a wet humidifiedcarbon dioxide (“CO₂”) environment are being considered, and appear toprovide a compromise between current-carrying capability and long-termwear:

-   -   “Brushes are the biggest challenge for DC homopolar machines.        The brushes are located in the stationary part of the motor        (stator) and provide the electrical connection to the normally        conducting, liquid-cooled rotor. Solid carbon or metal-graphite        brushes were found inadequate due to their low current density        capability and excessive wear. Graphite fiber brushes were used        with some success in early superconducting homopolar motors.        Liquid metal brushes were also developed and applied to        homopolar machines through the 1980s, but were not suitable for        many applications due to material and life limitations.        Currently copper fiber brushes operating in a wet humidified CO₂        environment are being considered, and this provides a compromise        between current-carrying capability and long-term wear. Such        brushes are expected to provide a five-year operational life.        For a typical operating profile this equates to motor slip-ring        travel of 6.5×10⁷ m/yr. A significant challenge is to control        the brush losses in order to limit their maximum operating        temperature. For this reason[,] complex means for brush loading        and liquid cooling of the rotor and stator are necessary.”

Table 4, below, contains wear data taken from Table 3 of US 2014/0045348A1, supra, compiled for improved FOT brushes (i.e., with the centermostfibers removed) which were tested successfully for 4.22×10⁹ and 5.5×10⁹inches of ring travel with cantilever and negator springs, respectively.It was concluded that, based on the condition of the brushes, the testresults could be projected for another 5-10 billion inches of ringtravel. These tests were conducted at ambient temperature with a verythin film of lubricant. It would be expected that an equal or betterlife would be achieved when running submerged in liquid nitrogen. Thefree length of the brushes was 0.4 inches at the beginning of test.Based on the results of this test, the requirement of 6.5×10⁷ m/yr.(2.56×10⁹ inches/yr.), referenced above, should be exceeded by a factorof two.

TABLE 4 Small Diameter Rotor Wear Study Cantilever Spring Negator SpringTotal inches of travel 4.22 × 10⁹  5.5 × 10⁹  Total wear (inches) 0.0250.010 Dimensionless wear rate 5.92 × 10⁻¹² 1.82 × 10⁻¹² (inches/inch)

The advantages of improved FOT brushes (i.e., with the centermost fibersre-moved) are summarized in Table 5.

TABLE 5 Advantages of Improved FOT Brushes 1 Number of fibers in thebundle can be varied to meet the power requirements. 2 Center can beremoved from the brush to give higher compliance, less frictionalheating, and longer wear life. (See Table 4.) 3 Collimator can beadjusted to vary free length of fibers to facilitate long wear life. 4FOT brush and tube can be designed with an adjustable collimator thatwill limit brush fiber distortion in response to B-field. 5 FOT brushcross-section is circular and, therefore, B-field forces are the samefrom all directions and, thus, is superior to a high-aspect brush. 6Transfer of gold (“Au”) from the ring to silver/copper (“Ag/Cu”) fibersgives a more-noble contact interface and a lower contact resistance,which reduces the consumption of liquid nitrogen. (See FIGS. 1A-1D andFIGS. 4A-4D.) 7 As transfer continues, the contact interfacework-hardens, and, thus, gives a longer contact life. 8 Superconductingribbons located on both sides of the brush-springs stabilize thedeflection of the spring when cooled to liquid nitrogen temperature. 9 Aslip-ring typical of that shown in FIG. 5 demonstrated the ability tocarry 21,230 amps/sq.-in. current density with an improved FOT brush(1200 fibers) at liquid nitrogen temperature. (See FIG. 6.) 10 AnodeEffect When DC power is being transmitted by a sliding contact, thepositive brush can wear as much as 10 times more than the negative andneutral brushes. The dielectric strength of liquid nitrogen is higherthan the dielectric strength of air by a factor of about 20, and, thus,should reduce the difference in wear rate between anode and cathode

In many cases, superconductors like those referenced above, and also inTable 6, below, have been used. [Source: Sheahen, Thomas P.;Introduction to High-Temperature Superconductivity; Plenum Press, NewYork; (1994), Chapter 1, p. 4]:

-   -   “There are several ceramics, based on copper oxide, which remain        superconducting near 100 K. For example, the compound yttrium        barium copper oxide (YBCO) has been found to be superconducting        up to 92 K. This may not seem like a ‘high’ temperature to most        people, but to the engineers figuring the cost of refrigerants,        it is high enough: liquid nitrogen is sufficient to cool YBCO        into its superconducting range.”

Superconductivity is a phenomenon exhibited by certain materials whencool-ed below their respective transition temperatures. Below thesetransition temperatures, the superconducting materials exhibitsubstantially-zero electrical resistance. Early efforts in this regardwere limited by the temperatures needed to cause various materials totransition to superconducting status. For example, helium becomes liquidat about 4.2° K, neon becomes liquid at about 28° K, nitrogen becomesliquid at about 77° K, and oxygen becomes liquid at about 90° K.However, oxygen is combustible. It is easier and much less costly toliquefy nitrogen, than it is to liquefy helium and neon. At the sametime, newer materials have been developed that have higher transitiontemperatures below which they become superconducting. These are known ashigh-temperature superconducting (“HTSC”) materials. Examples of suchHTSC materials are set forth in Table 6.

TABLE 6 HTSC Materials Transition Temperature Name Formula (° K) Yttriumbarium copper oxide Y₁Ba₂Cu₃O₇ 92 Bismuth strontium calcium (Bi,Pb)₂Sr₂Ca₂Cu₃O_(x) 105 copper oxide (2223 phase) Thallium bariumcalcium, copper Tl₁Ba₂Ca₂Cu₃O_(y) 115 oxide (1223 phase) Mercury bariumcalcium copper Hg₁, Ba₂Ca₂ Cu₃O_(y) 135 oxide (1223 phase)

Accordingly, it would be highly desirable to provide improved electricalcontacts for transmitting electrical power and/or signal(s) between arotor and a stator.

It would also be highly desirable to provide improved fiber brushassemblies for use in slip-rings and homopolar motors and generators.

It would also be highly desirable to provide slip-rings and homopolarmotors/generators that employ FOT brush technology combined withsuperconducting ribbons strategically located on FOT brush-springs, onthe inner diameter of the rotor ring, and on the disk-armature of ahomopolar motor to significantly increase the current-carryingcapability of the brush-spring and ring combination, when cooled belowthe transition temperature(s) of the superconducting material(s). Thesuperconducting ribbon is located on the inner diameter of the rings(slip-ring) and disk-armature (homopolar motor) so that the FOT brusheswill not wear through the copper layer on the ribbon. In this case, theliquid nitrogen that reduces the brush and ring combination temperatureto 77° K, will also be the lubricant for the contact interface.

These and other objects and advantages will become apparent from theforegoing and ongoing written specification, the drawings, and theappended claims.

DISCLOSURE OF THE INVENTION

With parenthetical reference to the corresponding parts, portions orsurfaces of the disclosed embodiment(s), merely for purposes ofillustration and not by way of limitation, the present inventionprovides an improved device adapted to provide electrical contactbetween a stator and a rotor.

In one aspect, the improved device (40) is adapted to provide electricalcontact between a stator and two rotor rings (41A, 41B, . . . ), andbroadly includes: at least two current-carrying brush-springs (34, 34)mounted on the stator, each brush-spring having two opposite surfaces;at least one fibrous brush assembly (35) mounted on each brush-spring,each brush assembly having a bundle of fibers (36) arranged such thatthe tips of the fibers will engage an associated rotor ring fortransferring electrical current between the stator and the associatedrotor ring; a ribbon (32, 33) of superconducting material mounted onopposite surfaces of each brush-spring and communicating the stator witheach brush assembly mounted on the associated brush-spring; anotherribbon (29) of superconducting material mounted on the inside diametersof the rotor rings; and wherein the device is submerged in a cryogenicfluid at a temperature below the transition temperatures of thesuperconducting materials such that the electrical resistivity of thedevice will be reduced to substantially-zero and thecurrent-transferring capability of the device will be increased.

The dielectric strength of the cryogenic fluid may be about twenty timesthe dielectric strength of air.

The superconducting materials (29, 32, 33) may be the same.

The cryogenic fluid may be liquid nitrogen.

The ribbons mounted on each brush-spring are secured thereto, as bybeing soldered to the brush-spring.

The ribbons (32, 33) mounted on each brush-spring (31) may be configuredand arranged such that the force exerted by each brush-spring on theassociated brush assembly will not be affected by the superconductingribbons mounted thereon when the device is cooled to the temperature ofthe cryogenic fluid.

The ribbons (32, 33) mounted on each brush-spring may be dimensionallythe same.

Each brush assembly may include a brush tube (37), and one marginal endportion of the associated fiber bundle may be received in the brushtube.

The device may further include: a collimator tube surrounding a portionof each brush tube and extending therebeyond such that the lower end ofthe collimator tube is adapted to limit lateral movement of the lowermarginal end portions of the fibers in the associated bundle when therotor rotates relative to the stator.

Each collimator tube may be adjustably mounted on the associated brushtube.

A central portion of the fibers below each brush tube may be removed sothat the fibers extending below each brush tube are in the form of anannulus.

The device may be a slip-ring (40), a homopolar motor (80), a homopolargenerator (80), or some other type of device.

The fibers in the bundle have a nominal diameter of about 0.003 inches,and the maximum current density per fiber may be about 1769 amps/sq.-in.

In another aspect, the invention provides an improved method ofproviding a gold-on-gold electrical sliding contact between a rotor ring(41) and the tips of a plurality of metal fibers (36) mounted in a brushassembly (35) on a stator, comprising the steps of: providing a rotor(41); providing a gold ring on the rotor; providing a brush assembly(35) having a bundle of silver/copper fibers (36); mounting each brushassembly on the stator such that the tips of the metal fibers engage therotor ring; submerging the brush assemblies in a cryogenic fluid; andmoving the rotor relative to the fiber tips such that gold istransferred from the rotor ring to the tips of the fibers; thereby toprovide a gold-on-gold electrical sliding contact between the rotor ringand the tips of the fibers.

The rotor may be part of a drum-type slip-ring (see, e.g., FIG. 5), apancake-type slip-ring (see, e.g., FIG. 7), a disk-armature-type ofhomopolar motor or a disk-armature-type of homopolar generator (see,e.g., FIG. 9). The rotor ring may be provided on an outer surface of therotor.

The method may include the additional steps of: providing acurrent-carrying brush-spring (31) having opposite surfaces; providingribbons (32, 33) of superconducting material; mounting a ribbon of thesuperconducting material on each of the brush-spring opposite surfaces;mounting a proximate end of the brush-spring on the stator; and mountingeach brush assembly (35) on a distal marginal end portion of thebrush-spring such that the stator communicates with the brush assemblythrough the brush-spring and the ribbons.

The method may further include the additional steps of: providinganother ribbon (29) of superconducting material; and mounting such otherribbon on the inside diameter of the rotor ring (41).

The cryogenic fluid is preferably at a temperature less than thetransition temperature of the superconducting material, and the othersuperconducting material.

The step of moving the rotor ring relative to the fiber tips may includethe step of rotating the rotor ring relative to the fiber tips.

The method may further include the additional step of: biasing the tipsof the metal fibers against the rotor ring with a force of about 200grams.

In another aspect, the invention provides an improved slip-ring (40)which includes: a rotor (41 a, 41B, . . . ) having at least two goldrings; a stator; spring assemblies (31, 31) having proximal ends mountedon the stator and having distal ends arranged proximate an associatedone of the rotor rings, each spring assembly including a superconductingmaterial (32, 32); at least one brush assembly (35) mounted on thedistal marginal end portion of each spring assembly, each brush assemblyhaving a brush holder (37) and having a plurality of silver/copper metalfibers (36) arranged in a bundle, one marginal end portion of the bundlebeing arranged within the associated brush holder, the opposite end ofthe bundle terminating in a plurality of fiber tips engaging suchassociated rotor ring; wherein the slip-ring is submerged in a cryogenicfluid such that the temperature of the superconducting material isreduced below its transition temperature; and wherein the tips of thesilver/copper fibers are covered with gold; whereby the slip-ring willhave gold-on-gold sliding contact between the tips of the fibers and therotor ring.

In another aspect, the invention provides an improved homopolar motor(80), which includes: a rotor (83) having at least one gold ring (88); astator; at least one spring assembly (82), each spring assembly havingits proximal end mounted on the stator and having its distal endarranged proximate an associated rotor ring, each spring assemblyincluding a superconducting material (85); at least one brush assembly(69) mounted on a distal marginal end portion of each spring assembly,each of the brush assembly having a brush holder (72) and having aplurality of silver/copper metal fibers (71) arranged in a bundle, onemarginal end portion of the bundle being arranged within the associatedbrush holder, the opposite end of the bundle terminating in a pluralityof fiber tips engaging the associated rotor ring; wherein the homopolarmotor is submerged in a cryogenic fluid such that the temperature of thesuperconducting material is reduced below its transition temperature;and wherein the tips of the silver/copper fibers are covered with gold;whereby the homopolar motor will have gold-on-gold sliding contactbetween the tips of the fibers and the rotor ring.

In still another aspect, the invention provides an improved homopolargenerator (80), which includes: a rotor (83) having at least one goldring (88); a stator; at least one spring assembly (82), each springassembly having its proximal end mounted on the stator and having itsdistal end arranged proximate an associated rotor ring, each springassembly including a superconducting material (85); at least one brushassembly (69) mounted on a distal marginal end portion of each springassembly, each of the brush assembly having a brush holder (72) andhaving a plurality of silver/copper metal fibers (71) arranged in abundle, one marginal end portion of the bundle being arranged within theassociated brush holder, the opposite end of the bundle terminating in aplurality of fiber tips engaging the associated rotor ring; wherein thehomopolar motor is submerged in a cryogenic fluid such that thetemperature of the superconducting material is reduced below itstransition temperature; and wherein the tips of the silver/copper fibersare covered with gold; whereby the homopolar generator will havegold-on-gold sliding contact between the tips of the fibers and therotor ring.

Therefore, the general object of the invention is to provide improvedsuperconducting devices for providing electrical contact between tworelatively-movable members, such as a rotor and a stator.

Another object is to provide improved superconducting devices for use asa slip-ring, a homopolar motor, a homopolar generator, or the like.

These and other advantages will become apparent from the foregoing andongoing written specification, the drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a SEM photograph looking at the tips of a prior art fiberbrush.

FIG. 1 B is an EDAX analysis of the tips of the fibers, taken within theindicated box in FIG. 1A, noting the presence of gold on the fiber tips,in addition to silver and copper.

FIG. 1C is an EDAX analysis of the fiber shown in FIG. 1D, confirmingthat the fiber is made of an Ag/Cu alloy.

FIG. 1D is an SEM photograph of an Ag/Cu fiber as a material reference.

FIG. 2 is a plot of static contact resistance vs. current for two Ag/CuFOT brushes on a silver-plated rotor ring, and also for an Ag/Cu FOTbrush on a gold-plated rotor ring.

FIG. 3 is a schematic isometric view of a test apparatus withsmall-amplitude rotor oscillation capability for making FOT staticresistance measurements, with the brush and ring submerged in a pool(not shown) of liquid nitrogen.

FIG. 4A is a SEM photograph looking at the tips of a high-complianceAu/Cu FOT brush.

FIG. 4B is an EDAX analysis of the tips of the fibers, taken within thebox indicated in FIG. 4A, noting the presence of gold on the fiber tips,in addition to silver and copper.

FIG. 4C is an EDAX analysis of the fiber shown in FIG. 40, confirmingthat the fiber is made of an Ag/Cu alloy.

FIG. 40 is an SEM photograph of an Ag/Cu fiber as a material reference.

FIG. 5 is a schematic isometric view of a drum-type multi-channelsuperconducting slip-ring employing the principles of the presentinvention

FIG. 6 is a plot of voltage drop vs. current and contact resistance vs.current with high-compliance FOT brushes rotating at 30 rpm whensubmerged in liquid nitrogen and for currents ranging from 0-180 A.

FIG. 7 is a schematic top plan view of a pancake-type multi-channelsuperconducting slip-ring employing the principles of the presentinvention.

FIG. 8 is a schematic view of an improved drum-type slip-ring withsuperconducting ribbons attached to a cantilever brush-spring and insidediameter of the rotor.

FIG. 9 is a schematic view of an improved disk-armature homopolarmotor/generator provided with FOT brushes and superconducting cantileversprings.

FIG. 10 is a schematic view of an improved superconducting drum-typeslip-ring with cantilever brush-springs and additional FOT brushassemblies for additional current capacity.

FIG. 11A is a schematic view of a superconducting drum-type slip-ringwith a negator spring and an FOT brush assembly.

FIG. 11B is a detail view showing the superconducting ribbon on the leadcommunicating with the superconducting ribbon in the center of the metalbrush holder.

FIG. 12A is a schematic view of an improved superconducting drum-typeslip-ring having a negator spring and an FOT brush assembly mounted ondistal end of cantilevered spring.

FIG. 12B is a detail view showing the superconducting ribbon on the leadcommunicating with the superconducting ribbon in the center of the metalbrush holder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At the outset, it should be clearly understood that like referencenumerals are intended to identify the same structural elements, portionsor surfaces consistently throughout the several drawing figures, as suchelements, portions or surfaces may be further described or explained bythe entire written specification, of which this detailed description isan integral part. Unless otherwise indicated, the drawings are intendedto be read (e.g., cross-hatching, arrangement of parts, proportion,degree, etc.) together with the specification, and are to be considereda portion of the entire written description of this invention. As usedin the following description, the terms “horizontal”, “vertical”,“left”, “right”, “up” and “down”, as well as adjectival and adverbialderivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”,etc.), simply refer to the orientation of the illustrated structure asthe particular drawing figure faces the reader. Similarly, the terms“inwardly” and “outwardly” generally refer to the orientation of asurface relative to its axis of elongation, or axis of rotation, asappropriate.

Transfer Of Gold From Rotor Ring To Fiber Tips In An ExperimentalSuperconducting Sip-Rind (FIGS. 1A-1D)

Referring now to the drawings, and, more particularly to FIGS. 1A-1Dthereof, these four drawing figures depict a Scanning ElectronMicroscope (“SEM”)/Energy Dispersive X-Ray (“EDAX”) analysis of a priorart brush bundle shown in FIG. 1A

The individual fibers in the bundle are formed of a silver/copper(“Ag/Cu”) alloy. The fibers were mounted on a stator, were submerged inliquid nitrogen, and were arranged such that the tips of the fibersslidably engaged an electrodeposited layer of gold on a rotor. FIG. 1Ais a SEM photograph of the distal ends of the fibers in the bundle, andshows the bundle as having a circular FOT configuration (i.e., withoutthe centermost fibers removed). FIG. 1B is an EDAX analysis of the fibertips, taken within the indicated box in FIG. 1A. FIG. 1B indicates thepresence of gold on the fiber tips, in addition to silver and copper.Since the only source of gold is the electrodeposited layer of gold onthe rotor, FIG. 1B indicates that gold from the electrodeposited rotorring has been transferred from the rotor ring to the tips of the fibersin the bundle in the experimental superconducting slip-ring.

[FIG. 1C is an EDAX analysis of an intermediate portion of an Ag/Cufiber as a material reference. FIG. 1 D is an SEM photograph of theintermediate portion of the fiber shown in FIG. 1 D. The presence ofgold on the tips of the silver/copper fibers (FIGS. 1A-1B) confirms thatgold has been transferred from the rotor ring to the tips of the fibers.

Electrical Contact Resistance v. Current (FIG. 2)

FIG. 2 is a plot of static FOT contact resistance (ordinate) vs. current(abscissa) for the stator-mounted superconducting brush-spring and rotorring, partially shown in FIGS. 1A-1D, when submerged in a pool (notshown) of liquid nitrogen. The contact resistance is displayed inmilliohms, and the current is displayed in amps. The two curvesconnecting the square- and diamond-shaped data points depict the valuesobtained for an Ag/Cu FOT brush riding on a silver-plated rotor ring.The curve connecting the triangular data points depicts the valuesobtained for an Ag/Cu FOT brush riding on a gold-plated rotor ring.

These curves show that the contact resistance of the Ag/Cu FOT brushesriding on the silver-plated rings decreases sharply with increasingcurrent between 0-30 amps, and then begins to level out (i.e., to fallless sharply) between 30-70 amps. On the other hand, the contactresistance of the Ag/Cu FOT brush riding on the gold-plated rotor ringis substantially constant in the entire current range between 0-70 amps.This confirms the nobility of the Ag/Cu fibers with gold tips sliding ona gold ring.

Drum-Type Single-Channel Test Apparatus (FIG. 3)

FIG. 3 is a schematic view of a test apparatus for making the static FOTcontact resistance measurements shown in FIG. 2, with the brush and ringsubmerged in a pool (not shown) of liquid nitrogen. This apparatus is asingle-channel drum-type slip-ring that was designed to undergosmall-amplitude oscillations, but not rotate.

This apparatus, generally indicated at 20, has an upwardly-facingcup-shaped rotor 21 arranged to be oscillated at small amplitudes ineither angular direction, as indicated by the bidirectional arrows onarcuate line 22, about the axis (y-y) of a central vertical cylindricalshaft 23. The rotor is shown as having a superconducting ribbon 29attached to its inside diameter 26, and has a lead 30. The rotor has anannular horizontal base 24 connected to the shaft. The ring 25 hasinwardly- and outwardly-facing cylindrical surfaces 26, 28,respectively, that are generated about the axis of shaft 23.

A stator-mounted current-carrying brush-spring 31 has onesuperconducting ribbon 32, and the assembly thus formed is generallyindicated at 34. Conductor 31 is shown as being a horizontally-elongatedbar-like member having a rectangular transverse cross-section. Thesuperconducting ribbon 32 is shown as being a horizontally-elongatedbar-like member also having a rectangular transverse cross-section. Theribbon 32 may be suitably secured to the facing surface of conductor 31,as by soldering or the like. The proximal marginal end portions of theribbon-and-conductor subassembly 34 are mounted on a stator (not shown).The ribbon 32 is connected via a lead 38. A suitable voltage source(±ΔV) is applied between leads 30, 38 to cause current to flow from thesource through the slip-ring and back to the source.

A fibrous brush assembly 35 is mounted on the distal marginal endportion of the ribbon-and-conductor assembly 34. The brush assemblyincludes a fibrous brush 36, comprising a plurality of metal fibers 36,mounted within a brush tube 37.

The entire apparatus 20 is submerged in a pool (not shown) of a suitablecryogenic liquid, such as liquid nitrogen. While the ribbon 32 has someelectrical resistance at room temperature, when cooled below thetransition temperature (as when submerged in liquid nitrogen), it hassubstantially-zero electrical resistance. Since electrical current isconveyed from the stator to the brush assembly 35 along theribbon-and-conductor subassembly 34 through the path of least electricalresistance, the submerged ribbon-and-conductor assembly 34 can carryhigh electrical currents between the stator and the brush assembly alongthe superconducting ribbon 32 at substantially-zero electricalresistance. After passing through the radial thickness of the rotor,such transferred current may be conveyed by the superconducting ribbon29 on the inside of the rotor ring and returned to the sources.

Transfer of Gold from Rotor Ring to Fiber Tips In A SuperconductingSlip-Ring Using High-Compliance Improved FOT Brushes (FIGS. 4A-4D)

FIGS. 4A-4D depict an SEM/EDAX analysis of a superconducting slip-ringusing a high-compliance brush bundle (i.e., with the centermost fibersremoved).

As with the prior art brush bundle shown in FIG. 1A, the individualfibers in the high-compliance brush bundle shown in FIG. 4A are againformed of a silver/copper alloy. The brush bundle was mounted on astator, was submerged in liquid nitrogen, and was arranged such that thetips of the fibers slidably engaged an electrodeposited layer of gold onthe rotor. FIG. 4A is an SEM photograph of the distal ends of the fibersin the bundle, and shows the bundle as having an improvedhigh-compliance FOT configuration (i.e., with the centermost fibersremoved). FIG. 4B is an EDAX analysis of the fiber tips, taken withinthe indicated box in FIG. 4A. FIG. 4B indicates the presence of gold onthe fiber tips, in addition to silver and copper. This confirms thatgold from the electrodeposited rotor ring has been transferred from therotor ring (i.e., the only source of gold) to the tips of thesilver/copper fibers in the bundle.

FIG. 4C is an EDAX analysis of an intermediate portion of an Ag/Cu fiberas a material reference. FIG. 40 is an SEM photograph of an intermediateportion of the fiber. The presence of gold on the tips of thesilver/copper fibers (FIGS. 4A-4B) confirms that gold has beentransferred from the rotor ring to the tips of the fibers.

Drum-Type Multi-Channel Superconducting Slip-Rind (FIG. 5)

In FIG. 5, a drum-type multi-channel superconducting slip-ring,generally indicated at 40, is shown as having a plurality ofvertically-stacked rings 41 separated by intermediate dielectricbarriers, which have shoulders to support a ring on each side. Thevarious rings are severally indicated at 41 and are individuallyidentified by the letters “A”, “B”, “C” and “D”, respectively. Thus, theuppermost ring is indicated at 41A, the next-lower ring is indicated at41B, the third-lower device is indicated at 41C, and the lowermost ringis indicated at 41D.

There is a plurality of vertically-spaced stator-mounted laminatedribbon-conductor-ribbon subassemblies 34, one for each device. Theindividual subassemblies 34 are individually identified by the letters“A”, “B”, “C” and “D”, respectively. Subassembly 34A is associated withuppermost ring 41 A, next-lower subassembly 34B is associated with ring41B, next-lower assembly 34C is associated with ring 41C, and lowermostassembly 34D is associated with lowermost ring 41D. While thesesubassemblies are as before described, the leads issuing therefrom areconnected to alternating DC voltage sources. Thus, the uppermost lead isconnected to a positive voltage source, the second lower lead isconnected to a negative voltage source, the third lower lead isconnected to a positive voltage source, and the fourth lower lead isconnected to a negative voltage source. The vertical spacing between theadjacent ribbon-conduc-tor-ribbon subassemblies 34 insures that each iselectrically isolated from its immediate neighbor.

Thus, the drum-type multi-channel slip-ring 40 in FIG. 5 is shown withfour rings 41A, 41B, 41C and 41D. The dielectric barriers are placed inthe spaces between the rings to electrically isolate each of the fourrings from its adjacent neighbor(s). Similarly, the variousribbon-conductor-ribbon assemblies 34 are vertically spaced from oneanother, and these are connected to alternating DC voltage sources.

The entire device 40 is submerged in a suitable cryogenic fluid, such asliquid nitrogen, to lower the temperature of the superconductingmaterial(s) below their respective transition temperature(s). Thus, thisdevice provides a drum-type multi-channel slip-ring with the electricalconnection between each rotor 41A, 41B, 41C, 41D and its associatedribbon-conductor-ribbon brush-spring assembly 34A, 34B, 34C, 34D,respectively, providing a separate channel that is isolated from theothers. Conductors 31 are shown as being horizontally-elongated bar-likemembers having rectangular transverse cross-sections. Thesuperconducting ribbons 32, 33 are shown as being horizontally-elongatedbar-like members having rectangular transverse cross-sections. Theribbons are preferably formed of the same superconducting material, andare dimensionally the same, so that the force exerted by theribbon-and-conductor spring assembly on the rotor will not be affectedby the presence of the superconducting ribbons when the device issubmerged in a cryogenic fluid.

Voltage Drop and Contact Resistance vs. Current (FIG. 6)

FIG. 6 is a plot of voltage drop (left ordinate) and contact resistance(right ordinate) vs. current (abscissa) for a device, such as shown inFIG. 5, with high-compliance FOT brushes. The voltage drop is expressedin volts, the contact resistance is expressed in milliohms, and thecurrent is expressed in amps. The tips of the fibers were urged toengage the ring on the outer surface of the rotor ring with 150 grams offorce.

The voltage drop vs. current is shown as being a straight line drawnthrough the square-shaped data points. This line is shown as extendingbetween one extreme point at 0.22 volts and 20 amps, and other extremepoint at 0.50 volts and 180 amps. Thus, the voltage drop risessubstantially linearly with increasing current.

The contact resistance v. current is shown by the curve connecting thetriangular data points. This curve is seen as falling sharply withcurrent from 5.6 milliohms at 20 amps to about 2 milliohms at about 70amps, and then leveling off (i.e., falling less sharply) in the range of70-180 amps until reaching a final value of about 1.4 milliohms at 180amps. Thus, whereas voltage drop increases substantially linearly ascurrent increases from 20 to 180 amps, contact resistance decreases(sharply at first, and then less sharply) over the same increasing rangeof current.

FIG. 6 (measurements at 30 rpm) show that the FOT brush contactresistance is in the milliohm range, and decreases as the current isincreased. Thus, the total cost of ownership is less than a brush withfewer points of contact and, thus, a higher contact resistance. Theapparatus for making these measurements is shown in FIG. 5.

Pancake-Type Multi-Channel Superconducting Slip-Rind (FIG. 7)

FIG. 7 shows how the principles of the present invention can beimplemented in a pancake-type multi-channel slip-ring.

In FIG. 7, the improved device, generally indicated at 50, is shown asincluding a rotor 51 adapted to be rotated about a vertical axis 52.Since FIG. 7 is a top plan view of the slip-ring, axis 52 isperpendicular to the plane of the paper, and is represented by a pointmarker. The rotor is a circular or annular member, when seen in topplan, and has a plurality of concentric rings. These rings are severallyindicated at 53, and are individually identified by the letters “A”,“B”, “C” and “D”, respectively, in a progression radially outwardly fromaxis 52. As previously noted, each ring is generated about rotor axis52.

Annular dielectric barriers, severally indicated at 58, are positionedin the walls of the rotor between adjacent rings to electrically isolateeach ring from its adjacent neighbor(s) and to prevent cross-couplingtherebetween. More particularly, there is a first dielectric barrier inthe wall between rings 53A, 53B, a second dielectric barrier in the wallbetween rings 538, 53C, and a third dielectric barrier in the wallbetween rings 53C, 53D.

A plurality of cantilever-mounted spring assemblies, severally indicatedat 59, and substantially as previously described, are arranged to rideon the various rings. Each of these springs is a ribbon-conductor-ribbonlamination, as described above, having their proximal marginal endportions mounted on the stator.

Brush assemblies, again severally indicated at 35, are mounted on thedistal marginal end portions of the springs 59. Each brush assembly hasa plurality of FOT-mounted brush fibers that are arranged to engage thesurface of the associated ring. There are two cantilever-mounted springsper ring, and these are arranged proximate the 12:00 o'clock and 6:00o'clock positions on the rotor. The springs associated with ring 53A areindicated at 59A, 59A; those associated with ring 53B are indicated at59B, 59B; those associated with ring 53C are indicated at 59C, 59C; andthose associated with ring 53D are indicated at 59D, 59D.

Like the drum-type embodiment which can have multiple brush assembliesmounted on each spring (see, e.g., FIG. 10), the pancake-type embodimentcan have multiple brush assemblies mounted on the distal marginal endportions of the springs to ensure adequate and continuous electricalcontact between the fiber tips and the bottom surfaces of the variousrings. Also, note that annular superconducting ribbons are located onoutside diameter and inside diameter of each ring.

Drum-Type Slip-Ring With Superconducting Ribbons Attached To OppositeSides Of A Cantilever-Mounted Brush-Spring And The Inside diameter OfThe Rotor (FIG. 8)

FIG. 8 is a schematic view of an improved drum-type slip-ring, generallyindicated at 60, which is operatively arranged to transmit electricalpower and/or signal(s) between a stator 61 and a rotor 62.

A cantilever-mounted spring assembly, generally indicated at 63, has itsleft or proximal end mounted on the stator 61, and has its right ordistal end arranged proximate the rotor. This spring assembly has acentral horizontally-elongated current-carrying brush-spring 64, and hasribbons 65, 65 of superconducting material mounted on the upper andlower surfaces of the central conductor. The central conductor may beformed of a suitable material, such as beryllium copper (BeCu), and hasa generally-rectangular transverse cross-section. Thus, the centralconductor has horizontally-elongated planar rectangular upper and lowersurfaces, 66, 68, respectively. The central conductor has two principalfunctions: (1) to support the superconducting ribbons which carry theelectrical current between its proximal and distal ends, and (2) to actas a flexure spring between its proximal and distal ends to urge thetips of a fibrous brush assembly 69 against a gold ring 70electrodeposited on the outer surface of the rotor. The superconductingribbons 65, 65 are mounted on the upper and lower surfaces of thecentral conductor. These two ribbons are horizontally elongated, arepreferably formed of the same superconducting material, and haveidentical rectangular transverse cross-sections. The ribbons aresuitably secured to the upper and lower surfaces of the centralconductor, as by soldering or the like. The principal function ofsuperconducting ribbons 65, 65 is two-fold: (1) to provide a path ofsubstantially-zero resistance to current flow from the stator to thebrush assembly when the entire apparatus is submerged in a cryogenicfluid to lower the temperature of the material of which ribbons 65, 65are formed below the its transition temperature, and (2) to cancel thetendency of the spring assembly to curl upwardly or downwardly (like abimetallic strip) when the apparatus is submerged in the cryogenicfluid.

The brush assembly 69 is mounted on the distal marginal end portion ofthe spring assembly. This brush assembly includes a bundle 71 of metalfibrous brushes therein. The individual fibers in this bundle may beformed of a silver/copper alloy. The upper marginal end portion of thisbrush bundle is received and held in a brush tube 72. The lower ends ofthe individual fibers in this brush bundle terminate in tips thatslidably engage the ring 70 on the outer surface of the rotor.

Rotor 62 is shown as being an annular member mounted for rotation aboutan axis 74. Since this axis comes out of the paper in FIG. 8, it isrepresented by a point marker. The rotor has a thin annular ring 70 ofgold electrodeposited on its outer cylindrical surface 75, and has aninner cylindrical surface 76. An annular superconducting ribbon 77 ismounted on the rotor inner surface 76. More particularly, ribbon 77 hasits outer cylindrical surface 78 suitably secured to the rotor innersurface 76, as by soldering, and has an inner cylindrical surface 79.

A voltage may be selectively applied between the proximal end of thespring assembly and the rotor, by suitable leads (not shown), such thatcurrent will be caused to flow from the stator to the rotor.

The entire apparatus is submerged in a suitable cryogenic fluid, such asliquid nitrogen, so as to lower the temperature of superconductingribbons 65, 65, 77 below their respective transition temperatures sothat these ribbons will have substantially-zero resistance to passage ofelectrical current therethrough.

Thus, when the apparatus is submerged in the cryogenic fluid, currentwill flow from the stator along the length of spring assembly 63 to thebrush assembly 69. Since ribbons 65, 65 will be below their transitiontemperatures, and will have substantially-zero resistance, these ribbonswill be the path of least resistance to passage of current between thestator and the brush assembly. After passing through the brush tube,current will pass downwardly along the length of the brush bundle, andwill be transferred from the tips of the fibers to rotor ring 70. Afterpassing through the rotor, current will be conveyed by ribbon 77, whichis also at substantially-zero resistance, to other parts (not shown) ofthe rotor.

Thus, in this embodiment, superconducting ribbons 65, 65, 77 provide thepath of least electrical resistance to passage of current from thestator to the rotor, when the slip-ring is submerged in cryogenic fluidbelow the transition temperature of the superconducting material(s).

Disk-Armature Homopolar Motor With FOT Brushes And SuperconductingCantilevered Springs (FIG. 9)

FIG. 9 is an isometric view of an improved disk-armature-type ofhomopolar motor or generator.

The improved motor, generally indicated at 80, has first and secondcantilever-mounted spring assemblies 81, 82 mounted on a stator (notshown). Each spring assembly has a proximal end mounted on the stator,and has a distal end arranged proximate a rotor, generally indicated at83. Each spring assembly is shown as being a horizontally-elongatedsandwiched structure having a central current-carrying brush-spring 84having upper and lower planar horizontal surfaces, with a ribbon 85 ofsuperconducting material secured thereto, as by soldering or the like.Thus, one ribbon 85 is secured to the upper surface of the centralconductor, and the other ribbon 85 is secured to its lower surface. Thecentral conductor may be formed of beryllium copper or the like, and isshown as having a rectangular transverse cross-section. Ribbons 85, 85are also shown as having rectangular transverse cross-sections. Thepurpose of a ribbon on both sides of the spring is to double the currentcapacity. The dimensions of these ribbons are preferably identical so asto cancel any tendency of the spring assembly to curl like a bimetallicstrip when submerged in cryogenic fluid below the transitionaltemperature(s) of the superconducting materials of which ribbons aremade. In FIG. 9, the two spring assemblies 81, 82 are shown as beingsubstantially identical. However, they need not necessarily be so.

The spring assemblies 81, 82 in FIG. 9 differ from the spring assembly63 in FIG. 8 in that there are two brush assemblies 69 mounted on thedistal marginal end portions of each spring assembly. As previouslydescribed, each brush assembly includes a brush tube 72, and a fiberbundle 71. The fiber bundle contains a plurality of individual fibersmade of a suitable silver/copper alloy. The upper marginal end portionof each fiber bundle is received and held in the associated brush tube,as described in U.S. Pat. No. 7,105,983 B2. The fibers in each bundleterminate in lowermost tips that slidably engage portions of the rotor,as described infra. The principal reason for two brush assemblies oneach spring assembly is to double the current capacity.

Rotor 83 is shown as being a horizontally-elongated specially-configuredmember journalled for rotation about horizontal axis x-x. Axis x-x isarranged orthogonally to the axes of elongation of spring assemblies 81,82. The rotor has a large-diameter disk with a shaft that is connectedto the disk by soldering or other acceptable method, such as threadswith thread lock. A plurality of circumferentially-spacedradially-extending superconducting ribbons is located on both sides ofthe disk. These ribbons are received in radially-extending slotsprovided in the disk. The thickness of the disk will be dictated by thewidth of the ribbon; for example, if the ribbon is ¼″ wide, the diskwill have to be thicker than % so that a ribbon can be inserted in theslots on either side of the disk. A thin annular or cylindrical ring 88of gold is electrodeposited on the outer surface of disk 88, and isslidably engaged by the brushes of second spring assembly 82.

The rotor also has a small-diameter shaft 89 that is a coupled to thelarge-diameter portion disk, and extends leftwardly and axiallytherefrom. A thin-walled annular ring 90 of gold is electrodeposited onthe left marginal end portion of small-diameter shaft 89, and isslidably engaged by the tips of the brushes in the first spring assembly81. A plurality of circumferentially-spaced longitudinally-extendingribbons of superconducting material, severally indicated at 91, connectthe inner ends of the several spokes with ring 90.

In use, a voltage (±ΔV) is applied between the proximal ends of springassemblies 81, 82, and a magnetic field, represented by arrows 84, 84,passes longitudinally through the rotor. The rotor rotates about axisx-x relative to the cantilever-mounted spring assemblies 81, 82, and thetips of the fibers in the respective brush assemblies slidably engagerotor rings 90, 88, respectively.

Current flows outwardly along spring assembly 81, down through the brushassemblies at the distal ends thereof, to rotor ring 90, is conveyedaxially along the rotor small-diameter shaft 89 by ribbons 91 to the hubspokes, is conveyed outwardly along the spokes to the superconductingribbon 86 of the large-diameter rotor portion, is conveyed through ring88 to the brushes of spring assembly 82, and is conveyed through springassembly 82 back to the stator.

The entire apparatus is submerged in a suitable cryogenic fluid, such asliquid nitrogen, to cool the apparatus down below the transitiontemperature(s) of the various superconducting material(s) therein. Whenso cooled, these various superconducting material(s) havesubstantially-zero electrical resistance and current takes the path ofleast resistance as it passes from the proximal portion of one springassembly to the proximal portion of the other spring assembly.

The direction of current flow is a function of the polarity of thevoltage (±ΔV) applied to the proximal end portions of spring assemblies71, 72.

Superconducting Drum-Type Slip-Ring With Cantilever-MountedBrush-Springs And Additional FOT Brush Assemblies For Additional CurrentCapacity (FIG. 10)

FIG. 10 is a schematic view of another embodiment of a drum-typesuperconducting slip-ring, generally indicated at 93.

The slip-ring shown in FIG. 10 is generally similar to that shown inFIG. 8, except that three brush assemblies, severally indicated at 69′,are mounted on the distal marginal end portion of a sandwiched springassembly 63 having superconducting ribbons 65, 65 secured to the upperand lower surfaces of a central conductor 64 to provide aribbon-conductor-ribbon spring assembly, as previously described.

Moreover, whereas the embodiment shown in FIG. 8 had one brush assembly69 mounted on the distal marginal end portion of spring assembly 63, inFIG. 10, there are three brush assemblies 69′, 69′, 69′. These brushassemblies 69′ differ from those previously described in that ribbons ofsuperconducting material, severally indicated at 94, are mounted on theoutside of the brush tubes 72′ so as to surround the upper marginal endportions of the brush bundles. Depending on the diameter of the brushtube, the superconducting ribbons may have to be narrow and mountedvertically so that the maximum bend radius is not exceeded. Moreover,the central cores of fibers in each brush bundle below the brush tubeshave been removed such that the portion of each bundle extendingdownwardly below the associated brush tube is annular. This type ofbrush bundle, and the advantages stemming therefrom, is disclosed andclaimed in US 2014/0045348 A1, which is assigned to the assignee of thepresent application and is incorporated by reference herein. The use ofsuch annular brush bundles affords greatly-enhanced life to theslip-ring.

As before, the entire slip-ring shown in FIG. 10 is adapted to besubmerged in a cryogenic fluid, such as liquid nitrogen, to reduce thetemperatures of the various superconducting materials below theirtransition temperatures.

Superconducting Slip-Ring With Negator Spring And FOT Brush Assembly(FIGS. 11A-11B)

FIG. 11A is a schematic view of an improved superconducting slip-ringhaving a modified brush holder, generally indicated at 95, operativelyassociated with a rotor 62, as previously described.

In FIG. 11A, the cantilevered spring has been replaced with modifiedbrush holder 95, which is similar to the brush holder shown in FIG. 12Aof US 2014/0045348 A1. To this end, the brush holder is adapted to bemounted on the stator (not shown), and has a lead 96 communicating witha brush block 98, which, in turn, is mounted for vertical slidingmovement relative to a tube 99. The brush block is biased to movedownwardly by a negator spring 100, which acts between the brush blockand the tube.

A plurality of fibrous brush assemblies, severally indicated at 101, ismounted on the brush block for movement therewith. Each brush assemblyincludes a fiber bundle. The upper marginal end portion of this bundleis received and held in the brush tube, and the lower marginal endportion of the bundle extends downwardly beneath the brush tube. Eachfiber in the bundle terminates at its lowermost end in a tip theslidably engages the ring on the rotor. The principal function of thenegator spring is to urge the brush holder to move downwardly withsubstantially constant force so as to compensate for wear.

The brush holder shown in FIG. 11A differs from that shown in US2014/0045348 A1 in that a superconducting ribbon 102 is arranged inparallel with lead 96, and the brush block is provided with asuperconducting ribbon 103 in its center. As best shown in FIG. 11B,ribbon 102 communicates with ribbon 103.

As before, the entire slip-ring is adapted to be submerged in a suitablecryogenic fluid to cool the superconducting ribbons to below theirrespective transition temperatures.

In FIG. 11A, the fibers in each bundle may be formed of a suitablesilver/copper alloy, and the lower marginal end portion of the variousbundles may be annular as taught by US 2014/0045348 A1.

Improved Slip-Ring Having Negator Spring And FOT Brush Assembly MountedOn Distal End Of Cantilevered Spring (FIG. 12)

FIG. 12A is a schematic view of another form of slip-ring assembly,generally indicated at 105.

In FIG. 12A, the lead 96 shown in FIG. 11 has been eliminated. Rather,the brush holder is shown as being mounted on the distal end portion ofspring assembly 63, as previously described. FIG. 12 B shows that lowerribbon 65 communicat3es with a superconducting ribbon 103 on the brushholder.

Here again, the entire apparatus is adapted to be submerged in asuitable cryogenic fluid to cool the various superconducting materialsto below their respective transition temperatures.

Modifications

The present invention contemplates that many changes and modificationsmay be made.

As noted herein, the principles of the invention may be applied toeither a pancake- or drum-type of slip-ring, and to a homopolar motor orgenerator. The brush bundles may be either compressed, as in the priorart, or annular, as taught by US 2014/0045348 A 1. The tips of thefibers may be urged against the rotor ring by a cantilever-mountedspring assembly, or by a negator spring, or by some other means.

The essence of the invention is to provide improved FOT contact with arotor ring, where certain portions of the apparatus are made ofsuperconducting materials such that the entire apparatus may besubmerged in a suitable cryogenic fluid, such as liquid nitrogen, tocool the superconducting materials below their respective transitiontemperatures. When this occurs, the superconducting materials will havesubstantially-zero electrical resistance.

Thus, the present invention involves the use of superconducting ribbonsto bring current to and from a sliding contact interface. Because oftheir high current density capability, FOT brushes can be configured toconduct current flowing in superconducting ribbons with low contactresistance. Low contact resistance is an extremely important factor forsuperconducting slip-rings and homopolar motors. The lower the contactresistance the less liquid nitrogen is consumed.

The superconducting ribbons may be ½-inch wide and have a criticalcurrent/_(c) of 307 amps. The critical current/_(c) is the current levelat which the self-field causes the superconducting state to collapse.[Source: US 2014/0045348 A1 published Feb. 13, 2014]:

-   -   “[0106] It is known that cantilever springs can be difficult to        work with because of mechanical instabilities. [See, e.g.,        Shobert, Erle; Carbon Brushes: The Physics and Chemistry of        Sliding Contacts; Chapter 4, FIG. 4.7, “Mechanical        Considerations in Brushes and Collectors”; (1965); at p. 87.]        [0107] “Chatter can take place on cantilever-spring brushes if        the spring can vibrate in a way that relieves the spring force        as the brush moves in one direction, and increases it in the        other. * * * This chatter can be minimized by (1) keeping the        brush as short as possible; (2) so designing the spring that it        is practically straight when under load; and (3) tapering the        spring, as shown in FIG. 4:7 b. Tapering decreases the        possibility that a natural period is available for resonant        vibration.    -   [108] In addition, a cantilever spring has the problem that the        brush force (F) decreases with brush wear (x), and ultimately        the life of the brush is limited by the minimum normal force        that is required to meet all electrical requirements. If there        is not adequate brush force, signal brushes will not operate at        acceptable electrical noise levels and power brushes may undergo        electrical arcing. This is a major factor for a brush that is        capable of billions of inches of ring travel. The negator spring        maintains a substantially-constant force over a given        displacement range throughout the life of the brush and,        therefore, the life of the brush is not limited by a decreasing        force with brush wear. Also, the negator spring provides an        inherent dampening mechanism and, therefore, brush spring        “chatter” is eliminated.”

The present invention includes the attachment of superconducting ribbonsto all components (i.e., both sides of the brush-springs, and theinsider diameter of the ring) for a slip-ring and drum-type armaturehomopolar motor. Attaching superconducting ribbons to each side of thebrush-spring increases the current capacity, as well as stabilizes thebrush-spring movement, and thus minimizes the brush force change whengoing from room temperature to liquid nitrogen temperature. When negatorspring designs are used as shown in FIGS. 11A-11B and FIGS. 12A-12B,tolerances must be allowed so that the brush holder and negator springare free to move over the broad temperature range. The negator springhas the advantage of applying a constant force to the brush over a broadrange of brush wear. The ribbons can be attached to both faces of adisk-armature homopolar motor. When the ribbons make the transition tothe superconducting state, the electrical resistance therein drops tozero and current flows without resistance until the critical current isexceeded. It is not practical to attach ribbons to the fibers in thebrush.

Recent studies have been conducted with FOT brushes and superconductingslip-rings. A drum-type slip-ring was configured with an input positivebrush and ring and an output negative brush and ring (see FIG. 5). Thefibers in each bundle have a nominal diameter of about 0.003 inches, andthe maximum current density per fiber is about 1769 amps/sq.-in. Acurrent density of 21,230 amps/sq.-in. was measured with a single FOTbrush with twelve-hundred 0.003″ diameter fibers (see FIG. 6).Additional brushes and ring pairs can be configured to carryproportional maximum current densities (see Table 7).

TABLE 7 Number of Current Density/Brush Fibers/Brush (amps/sq.-in.) 120021,230 2400 42,460 3600 63,690 4800 84,920As the numbers of fibers increases in the brush bundle, care must betaken to increase the diameter of the opening in the center of the brushas well as adjust the free length of the fibers so as to achieve thedesired brush compliance.

A pancake-type slip-ring is illustrated in FIG. 7. This geometry allowsa superconducting ribbon to be located on the inside diameter andoutside diameter of each ring, thus, doubling the current capacity whencompared to a drum-type slip-ring. An additional brush has been added toeach spring, which now increases the current capacity of the brushes tocompensate for the additional ribbon on each ring. The current capacityfor two brushes and springs per ring will be of the order of 360 amps,and four brushes per ring on the order of 720 amps, based on the datapresented in FIG. 6. Ribbons on the inside diameter and outside diameterof each ring will have a combined current capacity of 614 amps. Thus,the limiting factor for this configuration is not the brushes.

Therefore, while several forms and embodiments of the improved deviceshave been shown and described, and some modifications thereto have beenspecifically discussed, persons skilled in this art will readilyappreciate that various additional changes and modifications may be madewithout departing from the spirit of the invention, as defined anddifferentiated by the following claims.

1.-16. (canceled)
 17. A method of providing a gold-on-gold electricalsliding contact between a rotor ring and the tips of a plurality ofmetal fibers mounted in a brush assembly on a stator, comprising thesteps of: providing a rotor; providing a gold rotor ring on said rotor;providing a brush assembly having a bundle of silver/copper fibers;mounting each brush assembly on said stator such that the tips of saidmetal fibers engage said rotor ring; submerging said brush assemblies ina cryogenic fluid; and moving said rotor relative to said fiber tipssuch that gold is transferred from said rotor ring to the tips of saidfibers; thereby to provide a gold-on-gold electrical sliding contactbetween said rotor ring and the tips of said fibers.
 18. The method asset forth in claim 17 wherein said rotor is part of a drum-typeslip-ring, a pancake-type slip-ring, a disk-armature-type of homopolarmotor or a disk-armature-type of homopolar generator, and wherein saidrotor ring is provided on an outer surface of said rotor.
 19. The methodas set forth in claim 17, and further comprising the steps of: providinga current-carrying brush-spring having opposite surfaces; providingribbons of superconducting material; mounting a ribbon of saidsuperconducting material on each of said brush-spring opposite surfaces;mounting a proximate end of said brush-spring on said stator; andmounting said brush assembly on a distal marginal end portion of saidbrush-spring such that said stator communicates with said brush assemblythrough said brush-spring and said ribbons.
 20. The method as set forthin claim 19, comprising the additional steps of: providing anotherribbon of superconducting material; and mounting such other ribbon onthe inside diameter of said rotor ring.
 21. The method as set forth inclaim 19 wherein said cryogenic fluid is at a temperature less than thetransition temperature of said superconducting material.
 22. The methodas set forth in claim 17 wherein said cryogenic fluid is at atemperature less that the transition temperature of such othersuperconducting material.
 23. The method as set forth in claim 17wherein the step of moving said rotor ring relative to said fiber tipsincludes the step of rotating said rotor ring relative to said fibertips.
 24. The method as set forth in claim 17, and further comprisingthe additional step of: biasing the tips of said metal fibers againstsaid rotor ring with a force of about 200 grams.
 25. A slip-ring,comprising: a rotor having at least two gold rotor rings; a stator; aplurality of spring assemblies having proximal ends mounted on saidstator and having distal ends arranged proximate an associated one ofsaid rotor rings, each spring assembly including a superconductingmaterial; at least one brush assembly mounted on the distal marginal endportion of each spring assembly, each brush assembly having a brushholder and having a plurality of silver/copper metal fibers arranged ina bundle, one marginal end portion of said bundle being arranged withinthe associated brush holder, the opposite end of said bundle terminatingin a plurality of fiber tips engaging such associated rotor ring;wherein said slip-ring is submerged in a cryogenic fluid such that thetemperature of said superconducting material is reduced below itstransition temperature; and wherein the tips of said silver/copperfibers are covered with gold; whereby said slip-ring will havegold-on-gold sliding contact between the tips of said fibers and saidrotor ring.
 26. A homopolar motor or generator, comprising: a rotorhaving at least one gold rotor ring; a stator; at least one springassembly, each spring assembly having its proximal end mounted on saidstator and having its distal end arranged proximate an associated rotorring, each spring assembly including a superconducting material; atleast one brush assembly mounted on a distal marginal end portion ofeach spring assembly, each of said brush assembly having a brush holderand having a plurality of silver/copper metal fibers arranged in abundle, one marginal end portion of said bundle being arranged withinthe associated brush holder, the opposite end of said bundle terminatingin a plurality of fiber tips engaging said associated rotor ring;wherein said homopolar motor is submerged in a cryogenic fluid such thatthe temperature of said superconducting material is reduced below itstransition temperature; and wherein the tips of said silver/copperfibers are covered with gold; whereby said homopolar motor will havegold-on-gold sliding contact between the tips of said fibers and saidrotor ring.
 27. (canceled)